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On 29 July 2015, NASA released a second image from the Earth Polychromatic Imaging Camera (EPIC) on the Deep Space Climate Observatory (DSCOVR) satellite:

Second released EPIC image

If you zoom in on the full image, you will find a purple dot in the middle of the Egyptian desert:

Purple dot

This does not look physical, and it appears to be exactly in the middle. I don't find it in the first released EPIC image. It looks like some instrumental artefact. Why is it there?

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    $\begingroup$ I think it is there in the original image, but is black instead of purple. $\endgroup$
    – PearsonArtPhoto ♦
    Jul 29, 2015 at 15:18
  • $\begingroup$ @PearsonArtPhoto What do you mean by the original image? The raw monochrome images that went into the colour composite? Are those publicly released (yet)? Or do you mean the first released EPIC image on 20 July? $\endgroup$
    – gerrit
    Jul 29, 2015 at 15:19
  • $\begingroup$ I meant the first image. $\endgroup$
    – PearsonArtPhoto ♦
    Jul 29, 2015 at 15:21
  • $\begingroup$ Oh, I see the black dot in the 20 July composite image now. A purple dot could mean the area was white in some of the component images, but black in others. Hmm. $\endgroup$
    – gerrit
    Jul 29, 2015 at 15:21
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    $\begingroup$ I was really wondering if it was this: viralnova.com/… as it is visible from space and shows up quite black. $\endgroup$
    – Rory Alsop ♦
    Jul 29, 2015 at 17:34

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EPIC (PDF) is a Cassegrain type reflector telescope so there's the fixed hyperbolic secondary mirror in the middle of the telescopes light path / focal plane. While that could be removed during post-processing and combining multiple exposures focusing at slightly different angles of the telescope itself (or shifting of the sensor on the focal plane, depends on its exact design), it is also the region that would be most susceptible to even the slightest of deformations, or slight misalignment, possibly due to thermal cycling, of both the mirrors that are fixed and were collimated before flight:

Light path in a Cassegrain reflecting telescope

Additionally, due to DSCOVR's vantage point from Sun-Earth Lagrangian point 1 (SEL1), the telescope would focus on nearly 100% (99.6% of its great circle, to be precise) illuminated Earth with the Sun behind it. That ought to create a central region of coherent backscatter which would be difficult to remove even with multiple exposures, since it would be always present regardless of the telescope's slightly changing angle. So it would appear as an overexposed region, that might appear as pretty much anything after post-processing, including the Moiré fringed dot with typical read and blue channel interference pattern around it on the image that you noticed, once the RGB sources, or slightly different angles, or both, are interpolated. Notice that there's also a bit more of color bleeding around the central dot, than elsewhere on the image, with odd yellow-green hue patches for somewhere in the middle of the Saharan desert.

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    $\begingroup$ FWIW, there's nothing much of interest and of that size in the region where that spot appears (Just into Libya, West of Egyptian border). Moiré pattern removal tool removes color bands nicely without loss of sharpness, leaving an artifact resembling a spherical object roughly 40 km (25 mi) in diameter. It doesn't appear to be a dead pixel on the sensor, but a post-processing artifact of 5x5 pixels in the central region of a 2048x2048 pixel source. $\endgroup$
    – TildalWave
    Jul 29, 2015 at 17:18
  • $\begingroup$ As much of a fan as I am of all your great answers, I don't understand this one. How does a Cassegrain's secondary make a purple dot in the middle of the field? Perhaps it could be other optical elements in the path (e.g. lenses) that are actually generating flare (e.g. lens flare)? $\endgroup$
    – uhoh
    Oct 30, 2016 at 14:36
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    $\begingroup$ Both parts of the answer look like complete nonsense to me. Anyone who ever used a Newtonian telescope knows that the secondary mirror obstruction does not cause such effects. If the bright point is caused by "coherent backscatter", we should also see it regularly in photos taken from airplanes - but we don't. $\endgroup$
    – szulat
    Mar 9, 2017 at 22:45
  • $\begingroup$ It ought to be fairly obvious, the artifact isn't caused by backscatter, it's caused by overexposure of a spot that is 1) level, highly reflective, AND perfectly perpendicular to the camera's optical axis, 2) reflecting a tremendous amount of highly collimated light coming from the Sun located directly behind the camera, 3) yet the camera, being several billion orders of magnitude smaller than the diameter of the Sun, does not block any appreciable amount of the sun's energy falling on the spot in question. $\endgroup$
    – Michael C
    May 19, 2017 at 2:07

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